Traditionally, negative voltage charge-pumps are used to control radio frequency (RF) switches that are implemented using a pseudomorphic high electron mobility transistor/complementary metal oxide semiconductor (pHEMT/CMOS) process. Negative voltage generated by a negative voltage charge-pump is usable to turn off pHEMT/CMOS based RF switches. Using a negative voltage instead of a positive voltage improves the isolation of a pHEMT/CMOS based RF switch and eliminates the need for DC blocking capacitors for proper operation of pHEMT/CMOS based RF switches. The lack of blocking capacitors reduces the area needed to implement a pHEMT/CMOS based RF switch, which simplifies the design of electrostatic discharge (ESD) protection.
RF switches driven by a negative charge-pump typically have leakage currents that are supplied through a negative voltage output of the negative charge-pump. If an unregulated negative charge-pump is used to supply the leakage currents, an output voltage provided at the negative voltage output drops as the leakage currents increase. As a result, insertion losses and isolation numbers for the RF switches are adversely affected. In contrast, a regulated negative charge-pump will maintain a fixed output voltage as the leakage currents increase. It has been recognized that improved efficiencies can be realized by sourcing load currents in a feedback loop of the regulated negative charge-pump directly from a main power supply. For example, twice as much current will be drained from the main power supply if a load current is sourced from a voltage doubled section of the feedback loop. Such a large drain of current from the main power supply is inefficient and undesirable.
A regulated negative charge-pump will operate most efficiently when voltage regulation of the output voltage of the regulated negative charge-pump is maintained near to the regulated negative charge-pump's ideal output voltage. For example, an ideal output voltage for a regulated negative charge-pump that doubles a source voltage is twice the voltage of the source voltage. In general, a regulated negative charge-pump that is designed to boost a source voltage Vdd by a multiplication factor N will operate most efficiently when a negative output voltage VOUT is maintained at N times the source voltage Vdd. Efficiency for a charge-pump is given by the following equation:Efficiency=VOUT/(N*Vdd)
However, achieving a maximum efficiency depends on a given design specification and in some cases it might not be possible to regulate the negative output voltage VOUT close to an ideal voltage because the ideal voltage might be in the breakdown region of a CMOS switch being driven by the negative output voltage VOUT.
One way to implement a regulated negative charge-pump is through the use of a shunt regulator at the output of an unregulated negative charge-pump. FIG. 1 depicts a related art shunt regulated negative charge-pump 10 having a flying capacitor stage 12 with a capacitor CFLY having a first terminal that is selectively coupled between a first negative voltage source 14 through a first electronic switch 51 and a negative voltage output 16 through a second electronic switch S2. The capacitor CFLY also has a second terminal that is coupled to a fixed voltage node such as ground GND. The first electronic switch S1 is driven by a clock signal CLK, and the second electronic switch S2 is driven by a clock signal CLK. The clock signals CLK and CLK are non-overlapping and out of phase relative to each other. A positive regulated voltage source 18 is communicatively coupled to the negative voltage output 16 through a feedback network 20 made up of a first resistor R1 and a second resistor R2 coupled in a voltage divider configuration. An error amplifier 22 has an inverting input 24 coupled to a voltage divider node 26 between the first resistor R1 and the second resistor R2. The error amplifier 22 further includes a non-inverting input 28 that is coupled to a negative voltage reference −VREF.
The error amplifier 22 is powered by a second negative voltage source 32 that provides a negative supply voltage that is above the negative output voltage VOUT, and a third negative voltage source 34 that provides a negative voltage that is below the negative output voltage VOUT. A voltage of −3Vdd for the first negative voltage source 14 and the third negative voltage source 34 is generated by a negative voltage quadrupler (not shown). A voltage of −2Vdd for the second negative voltage source 32 is generated by a negative voltage tripler (not shown). The negative voltage tripler is able to triple the source voltage Vdd because a voltage equal to −Vdd is generated by a voltage doubler circuit (not shown) that is sourced from Vdd. Alternately, a cascading of two voltage doubler circuits can be adapted to provide a voltage tripler circuit.
The shunt regulated negative charge-pump 10 also includes an n-channel enhancement mode FET M1 that has a gate coupled to an error signal output 30 of the error amplifier 22. The FET M1 also has a source coupled to the third negative voltage source 34, and a drain coupled to the negative voltage output 16. A load capacitor CLOAD has a first terminal coupled to the negative voltage output 16, and a second terminal coupled to a fixed voltage node such as ground GND.
When the switch S1 is closed and the switch S2 is open, the flying capacitor CFLY is charged to −3Vdd. When the switch S2 is closed and the switch S1 is open, the FET M1 turns on and pulls charge away from the capacitor CFLY and the capacitor CLOAD until a voltage at the voltage divider node 26 is equal to the reference voltage −VREF. The load current ILOAD comes from the second negative voltage source 32, which in this case supplies a voltage of −2Vdd. As a result of the second negative voltage source 32 supplying the load current ILOAD at a voltage of −2Vdd, the source of the voltage Vdd must supply three times as much current as the load current ILOAD. Thus, the efficiency of the shunt regulated negative charge-pump 10 is less than the efficiency desired for battery powered equipment such as mobile terminals.
Another way to implement a regulated negative charge-pump is to use a series regulator at the output of an unregulated negative charge-pump. FIG. 2 depicts a related art series regulated negative charge-pump 36 that is similar to the shunt regulated negative charge-pump 10 (FIG. 1). However, in this particular case, the FET M1 is a p-channel enhancement mode type device that has a drain coupled directly to the second negative voltage source 32. In contrast to the shunt regulated charge-pump 10, the FET M1 has a source coupled to the negative voltage output 16. The source of the FET M1 is also coupled to the third negative voltage source 34 through a third resistor R3. However, even with these structural changes that implement series regulation in place of shunt regulation, the problem of inefficient operation remains. In effect, the load current ILOAD is still supplied by the second negative voltage source 32, which in this case supplies a voltage of −2Vdd. As a result of the second negative voltage source 32 supplying the load current ILOAD at a voltage of −2Vdd, the source of the voltage Vdd must supply three times as much current as the load current ILOAD. Thus, the series regulated negative charge-pump 36 also provides less than the efficiency desired for battery powered equipment such as mobile terminals. Therefore, a need remains for a high efficiency regulated negative charge-pump.